This invention relates to the field of power converter, in particular to the field of synchronous rectifier for high efficiency converters.
Power converter designs based on diode rectifiers are limited by the conduction loss of diode rectifiers due to their forward voltage drop, typically 0.7V for silicon diode, during forward conduction. This loss is significant when the rectified output voltage is low and comparable to the forward voltage drop of the diode rectifier. For example, the supply voltage for logic circuits nowadays and microprocessors can be as low as 2.2V or even lower in the future. The output diode rectifiers used in the converters for such applications typically consume one-third of the output power.
A known way to improve the rectification efficiency is to replace the diode rectifier by a synchronous rectifier using an active switch with low conduction loss like MOSFET. A synchronous rectifier has a lower forward voltage drop than a diode due to the much lower drop in a transistor. However, as an active switch a synchronous rectifier needs a driving signal in to turn it on at the appropriate times. In addition, the loss and performance of the active switch is sensitive to the driving signal amplitude and waveform. Consequently, the driving method becomes an important issue in synchronous rectifier design.
A typical synchronous rectifier makes use of a voltage signal derived from the main transformer windings to drive a MOSFET to ensure that the MOSFET turns on and off in synchronism with the alternating voltage signal on the transformer. However this driving method is not suitable for certain converter topologies. An example is the forward switching regulator using resonant reset. In this case the synchronous rectifier cannot obtain a driving signal during the entire conduction period because the driving voltage collapses with resetting of the main transformer. In the presence of the leakage inductance of the main transformer, no driving signal can be obtained during the commutation period. During this period the body diode instead of the conduction channel of the MOSFETs turns on for current conduction. This increases the losses in the synchronous rectifier especially at high frequency and high current because the forward voltage drop of the body diode is even higher than that of a conventional diode rectifier and with a further increase in the commutation time with the higher output current. Another example of a converter topology that is not driven well by transformer primary/secondary windings is the use of synchronous rectifier in low frequency AC rectification. The slow rising edge of sinusoidal driving voltage, e.g., 50 Hz or 60 Hz main transformer driven by a sinusoidal voltage, cannot efficiently drive the synchronous rectifier into its on state during the conduction period. These limitations impose restrictions on the input voltage range, the choice of topologies of the converter and particular applications.
Considerable effort has been expended in tackling the problem of efficiently driving a synchronous rectifier. U.S. Pat. No. 5,179,512, issued to Fisher et al. on Jan. 12, 1993, disclosed a gate drive circuit for synchronous rectifier. However, this gate drive can only work in resonant converters. U.S. Pat. Nos. 5,126,651 and 5,457,624, Kim R. Gauen (issued on Jun. 30, 1992) and Roy A. Hastings (issued on Oct. 10, 1995) respectively disclose drive circuits for synchronous rectifiers. These drive circuits can only be applied to non-isolated buck converter. Similarly, U.S. Pat. No. 5,303,138, issued to Allen F. Rozman on Apr. 12, 1994, disclosed gate drive circuits but without solving the problem of expanding the limited input voltage range. U.S. Pat. No. 5,097,403, issued to David A. Smith on Mar. 17, 1992, disclosed current sense rectifier and electronic circuits to detect current that are only applicable to MOSFET with current sense facility. Notably, U.S. Pat. No. 4,922,404, issued to Ludwig et al. on May 1, 1990, discusses the complexity of using a microprocessor to drive synchronous rectifiers. U.S. Pat. No. 6,134,131, issued to Poon et al. on Oct. 17, 2000, disclosed a current transformer for sensing the current and providing a suitable gate drive for the synchronous rectifier with current sense energy recovery. Although this design is superior in many respects, it is limited by the requirement that the current transformer should not experience saturation due to large operating duty cycle or low operating frequency. Moreover, noise may further interferes with the driving signal.
A method and system for improving synchronous rectifier performance with the aid of an additional hystersis driver is disclosed. This driver reduces noise interference with the driving signal, increases the operating frequency range, enhances the driving capabilities even with an otherwise too low a magnetizing inductance to sink the driving current to the gate of the MOSFET. The disclosed method and system overcomes problems due to saturation of the current sensing transformer in addition to producing a low magnetizing inductance resulting in greater flexibility in transformer design.
The disclosed method and ssytem encompasses efficient rectification of current in a selected branch of an electronic circuit. It makes use of a low loss MOSFET and with associated circuitry to realize the equivalent of a low loss diode with energy recovered from the current sensing means to ensure high efficiency.
In particular, the disclosed embodiments have a low loss active switching device with parallel diode such as a MOSFET, a plurality of windings, two diodes which are connected to a voltage source such as the output voltage or a zener diode. A first winding of the transformer is coupled in series with the diode simulating switching device. A second winding of the transformer is coupled to a hysterisis driver with its output coupled to the control terminal of the switching device. A third and a fourth winding of the transformer each with a series diode are connected to a voltage source.
Current flows through the first winding and a series MOSFET. A voltage is induced on the second winding and provides a driving signal for this MOSFET. The second winding is arranged to provide a positive voltage signal to the input of the hysterisis driver so that the MOSFET is driven ON for as long as possible while the current through the first winding flows in the forward direction.
A main current flowing through the first winding and the MOSFET produces a voltage on the second winding that, in turn, turns the MOSFET ON. However, this voltage may not be sustained throughout the period during which the current flowing through the primary winding is flowing in the forward direction. This is because magnetizing current increases with time and the voltage collapses when the magnetizing current exceeds the main current in the first winding. Therefore, increasing the time for which the MOSFET is ON improves the efficiency.
Use of a hysterisis driver is disclosed as one strategy to turn ON the MOSFET for a longer duration. The hysterisis driver overcomes this limitation because it has preset upper and lower thresholds. It turns on the MOSFET when the voltage induced on the second winding exceeds the upper threshold. Moreover, with the lower threshold set sufficiently low, the turn on signal is maintained as long as the main current remains positive. In other words even after the voltage on the second winding has collapsed the driving signal for turning the MOSFET ON is maintained. This ensures availability of a sufficient driving signal even when the current sensing transformer runs into saturation. Hence, the use of current driven technique results in the synchronous rectifier operating like a low loss active diode with the turning ON and OFF of the active switch independently of the input voltage.
The third winding limits the voltage generated and provides energy recovery. Voltage applied to the input of the hysterisis driver as well as the control terminal of the switching device must be limited to avoid damage to the switching device. The third winding of the transformer couples excessive energy to a voltage source and provides voltage clamping. The driving voltage amplitude is controlled by the turn ratio of the second to the third windings and the voltage source. A diode placed in series with the third winding ensures that voltage clamping is effective while the MOSFET is turned ON. This arrangement makes the driving signal independent of input voltage range and waveforms. Excessive energy in the first winding is transferred to the voltage source such as the DC output voltage of the power converter with the recovered current sensing energy becoming part of the output power.
The fourth winding provides magnetic reset. A reset mechanism is needed to allow an opposite voltage in the windings in the turn-off period after the transformer is energized during the turn-on period. The fourth winding provides a reset path whereby magnetizing energy stored in the transformer is released through a series connected rectifier to a voltage source. The phase of this fourth winding should be opposite to that of the third winding such that one winding facilitates the turn-on period while the other facilitates the turn-off or reset period. This arrangement allows the magnetizing energy to be recovered and reused.
Accordingly, the disclosed method and system provide an improved self-driven synchronous rectifier circuit with current sensing and suitable for wide input voltage and/or frequency ranges. In particular, sufficient driving signal is provided during current commutation along with energy recovery from current sensing. Moreover, the disclosed method and system have application to both isolating and non-isolating converters.